The present invention relates generally to automatic equalization of communication channels and, more particularly, to techniques for automatically reducing the effects of linear distortion in television channels.
The presence of linear distortion in a communication channel is a well known problem which deleteriously affects data transmission efficiency through the channel. In this regard, a band limited communication channel having a linear transmission characteristic is conventionally considered as being distortionless when characterized by a flat amplitude versus frequency response and linear phase response passing through zero degrees at zero frequency. Thus, any variance from these ideal channel behavioral responses will have the result of distorting the channel's transmission characteristics. In a television channel, for example, numerous effects are known to have an influence on the integrity of a video signal between its origin at the transmitter and its eventual application to a television picture tube for reproduction on the televised image. One common distortion producing effect is that of multipath transmission caused by the presence of various forms of signal reflectors in the transmission path of the television channel. As a consequence of the presence of these signal reflectors, the net signal collected at the television receiver antenna consists of the sum of the desired or main signal propagated between the transmitter and receiving antenna along a direct path and one or more reflected signals, commonly referred to as "echos", delayed in time and attenuated with respect to the main signal. In the television receiver, the reflected signals are transformed into electrical signals having substantially the same waveform as the main signal, but generally delayed in time and attenuated in amplitude therefrom. This effect is observably manifested by the production of replica images, normally referred to as "ghosts", displayed on the viewing screen from the image produced by the main signal. Ghost producing reflections of the foregoing type are commonly categorized as post-echos since they arrive at the television receiver antenna later in time than the main signal. Devices intended to remove such post-echos from the net signal collected by the television receiver antenna are usually referred to as ghost cancellation systems.
In addition to distortion resulting from post-echos caused by multipath transmission, imperfections in the video signal path of the television receiver itself also have a distorting influence on a processed video signal. Among other factors, antenna and tuner frequency response tilt as well as misalignment of the IF section of the receiver can result in imperfect channel transmission characteristics. Since these latter types of distortion are linear in nature, paired-echo theory indicates that they can be approximated in terms of appropriate sets of paired echos displaced in time about the undistorted transmitted signal. More specifically, the response of a linear distortion producing channel to an applied video signal is known to consist of the main undistorted transmitted signal plus at least one pre-echo similar in shape to the undistorted signal but preceding it in time by some amount T.sub.1 and a corresponding post-echo also similar in shape to the undistorted signal but following it in time by the amount T.sub.1. A full description and justification of this theory may be found in an article by H. A. Wheeler entitled "The Interpretation of Amplitude and Phase Distortion in Terms of Paired Echos", Proc. I.R.E. June, 1939, pages 359-385. The number and characteristics of the paired echo sets needed to approximate any particular linear distortion will, of course, depend upon the nature and severity of the distortion as well as the degree of accuracy to which the approximation is required. However, in any event, it is clear that all linear distortions, regardless of their origin, can be expressed as a sum of pre and ghost-echos of the undistorted signal. The generic process of removing distortion from a communication channel, including that of cancelling ghosts therefrom, is referred to as channel equalization and is implemented by devices known as channel equalizers.
It is significant to note that the major complication resulting from the extension of the problem of ghost cancellation to include that of full channel equalization is the requirement to include a facility whereby pre-echos in addition to post-echos may be cancelled. Thus, the problem of channel equalization, the cancellation of pre and post-echos, can be considered to broadly include that of ghost cancellation and a properly designed channel equalizer is therefore also capable of cancelling ghost producing post-echos from a video signal.
Conventionally, ghost cancellation devices employ the technique of delay and attenuation in a feedback or feedward path to eliminate the effect of post-echos caused by multipath transmission. In early devices, adjustable lengths of cable were used to appropriately delay a sample of the main signal which, after being attenuated and recombined with the main signal propagated through the receiver was effective for cancelling the later arriving ghost producing post-echo. In other words, a replica ghost signal is synthesized from the main signal and suitably delayed and attenuated such that, upon re-insertion in the video path, it is effective for cancelling the signal produced in the receiver in response to the later arriving post-echo. Employing generally similar principles more recently proposed ghost cancellation systems utilize charge-transfer devices (CTD) such as bucket-brigade devices (BBD) to effectuate the required delay in the sampled signal. The use of surface acoustical wave devices have also been proposed for this purpose. Exemplary systems of the foregoing type are disclosed in U.S. Pat. No. 3,935,536 issued Jan. 27, 1976 to Kimura et al. and U.S. Pat. No. 3,956,585 issued May 11, 1976 to Butler et al.
In addition to the attenuation and delay functions, an operable ghost cancellation system also includes means for appropriately setting the attenuators and delay lines to achieve cancellation of the existing post-echo signals. For this purpose, some prior art devices use manual means which are adjusted while the operator observes the effect on the displayed image. Manual setting techniques of this sort have not proven altogether satisfactory. Other systems use automatic setting techniques. Typically, in known automatic systems, signal processing circuits are utilized to define the post-echo signal or signals associated with a reference signal and to appropriately set the delay and attenuation necessary to eliminate the thusly defined post-echos. The horizontal synchronizing pulses of a transmitted television signal are sometimes used as the reference signal for this purpose.
Apparatus for achieving complete equalization of a video channel are also known in the art and, conceptually, are quite similar to the previously discussed ghost cancellation systems. Normally, a device, such as a transversal filter, capable of synthesizing replica signals of both the pre and post-echo components of a distorted video signal is automatically controlled for cancelling or equalizing the effect of the channel distortion represented by the pre and post-echos. Control of the transversal filter is typically accomplished by an adaptive control unit which is responsive to a broadcast reference signal. The reference signal comprises a signal whose time waveshape is standardized at the transmitter and stored in memory at the receiver and, preferably, includes spectral components covering the entire video bandwidth of approximately 4.2 megahertz thereby enabling equalization of the entire channel. In the control unit, an error criteria is employed to compare samples of the received reference signal with stored samples of what it should be for the case of undistorted transmission. The results of the comparison are then used to adjust the delays and attenuation of the replica signals developed by the transversal filter to minimize the distortion. Automatic equalization systems of the foregoing type are disclosed in articles by E. Arnon entitled, "An Adaptive Equalizer for Television Channels", IEEE Transactions on Communication Technology, Vol. COM-17, No. 6, December, 1969, page 726 and in an article by H. Rudin, Jr. entitled "Automatic Equalization Using Transversal Filters", IEEE Spectrum, January, 1967, page 53.
One significant problem associated with the use of automatic equalizers of the type discussed above is that the relatively high video base band frequencies of television signals preclude the use of much currently available signal processing technology. That is, according to Nyquist sampling theory, the highest frequency component which can be recovered from a sampled signal is one-half of the sampling frequency. Therefore, in order to recover the 4.2 megahertz components of the reference signal, thereby enabling equalization of the full channel bandwidth, the control unit must be capable of operating at a sampling rate of at least 8.4 megahertz. To satisfy the requirement of sampling the reference signal at a rate of at least 8.4 megahertz, prior art full bandwidth automatic equalizers for television channels have employed various sorts of specialized equipment all of which is relatively expensive and typically extensive in nature rendering their incorporation in commercial television receivers quite infeasible. In particular, analog elements such as operational amplifiers, integrators, differentiators, and multipliers are either unavailable at these frequencies (i.e. equal to or exceeding about 8.4 megahertz) or where available are prohibitively expensive. Digital techniques suffer the same difficulties. Digital to analog converters and analog to digital converters are either very expensive or do not operate at these frequencies. The microprocessor and related digital logic formulations, although quite attractive due to their potential low cost and computational capabilities, are especially limited in this respect. Partly for this reason prior art attempts to achieve computationally powerful and functionally adaptive ghost cancellation and channel equalization systems, particularly as part of a commercial television receiver, have been severely hampered. In particular, it would be highly desirable to provide a facility enabling the use of computationally powerful, although relatively slow speed, digital apparatus (e.g. a microprocessor) with relatively high video base band frequencies to achieve ghost cancellation as well as full bandwidth equalization of television channels. Considering the ever decreasing cost of digital logic, the inclusion of such systems in commercial television receivers provides the capacity for heretofore unrealized improvements in the quality of television signal reproduction.